1. Field of the Invention
The present invention is directed to an arrangement for tuning a balanced-to-ground resonator wherein the resonator includes at least one radio-frequency coil having an inductance which forms the resonant circuit for the resonator in combination with a capacitance.
2. Description of the Prior Art
Devices are known for producing tomograms of an examination subject, such as a human body, using the phenomenon of nuclear magnetic resonance. These so called NMR tomography devices include a fundamental field magnet system which aligns the nuclear spins in the examination subject, and also include gradient coils which generate a spatially different magnetic field, and radio-frequency coils for exciting the nuclear spins and for receiving the signals emitted as a result of the exited nuclear spins. Given the use of such a radio-frequency excitation and measuring coil, the inductance of the coil is connected in parallel with a fixed capacitor as an LC resonant circuit, and another capacitor (or capacitor arrangement) is then tuned to bring the circuit to the desired frequency. The use of a variable capacitor connected in parallel with the coil inductance and a series capacitor connected in the feed line for coupling the resonator to a radio-frequency generator is known for this purpose. The radio-frequency generator is preferably an oscillator having a following transmission amplifier. It is known to use a rotary capacitor as the variable capacitor, the capacitance of such an rotary capacitor being controlled by an electric motor, as described in Magnetic Resonance in Medicine 2 (1985), pages 490-494.
Depending upon the size of the examination subject, for example, a patient, the resonator is loaded to different degrees, i.e., damped. When matching the resonator to the radio-frequency source, which may have a characteristic impedance of for example, 50 ohms, one obtains matching frequencies which are dependent upon the degree of loading of the resonator. The natural frequency of the resonator must be detuned so that the frequency error caused by the different loading of the resonator is again compensated. After introduction of a patient into the nuclear magnetic resonance tomography device, tuning of the resonator is undertaken for this purpose by successively adjusting both the series capacitor for load matching and the parallel capacitor for frequency correction, until the required tuning is achieved.
It is known to use a differential rotary capacitor for tuning the resonator.
A structural embodiment of a capacitor for tuning an unbalanced-to-ground resonant circuit is described in U.S. Pat. No. 1,987,124. This structure includes two stacks of stator or fixed plates which are arranged side-by-side along the axial direction of a rotor shaft. As the shaft rotates, moveable or rotor plates are moved between the stator plates. The spacing in the stack of stator plates must be selected relatively large so that the residual capacitance between the two capacitors is maintained low. This residual capacitance reduces the range of variability in capacitances achievable by this structure.